This invention relates to an original document feeder for copying machines, and more particularly to a feeder for automatically positioning an original document on the exposure surface, comprising a plurality of friction wheels which rotate in order to feed an original document on to an exposure surface, and reference means which are selectively operable in order to align said documents in the feed direction.
From U.S. Pat. No. 4,170,414, an automatic document feeder for copying machines is known in which the documents are fed and aligned by motorised rollers pressed constantly against a slide surface in order to drag the documents one at a time against a mobile reference device for the purpose of correctly aligning the documents. a sensor disposed in proximity to the reference device halts the dragging rollers as soon as a document makes contact with the reference device. However, because of the inertia of the moving parts, the rotation of the rollers is not interrupted instantaneously when the sheet reaches the mobile reference device, but continues for a short period to cause the rollers to slide on the sheet. Now if the original sheet has a normal substance, i.e. from 60 to 80 g/m.sup.2, it has sufficient rigidity to resist the friction force applied by the rollers without bending. However, if the original sheets consist of very light sheets of paper, for example manifold paper having a substance of between 40 and 60 g/m.sup.2, with consequent low rigidity, they do not resist the friction force applied to them by the rollers, and thus wrinkle and bend, giving rise to inferior copies and ruining the original.